1. Field of the Invention
The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a fiber amplifier based light source for use with such inspection systems.
2. Description of the Related Art
The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Successive generations of such inspection systems tend to achieve higher resolution by illuminating the wafer or reticle using light energy having shorter wavelengths.
Certain practical advantages may be achieved when illuminating the wafer or reticle with light with wavelengths at or below 213 nm. Providing suitable lasers for high quality wafer and photomask inspection systems presents a particular challenge. Conventional lasers generating DUV light energy are typically large, expensive devices with relatively short lifetimes and low average power. Wafer and photomask inspection systems generally require a laser generally having a high average power, low peak power, and relatively short wavelength for sufficient throughput and an adequate defect signal-to-noise ratio (SNR).
The primary method to provide adequate DUV power entails generating shorter wavelength light from a longer wavelength source. This wavelength alteration process is commonly called frequency conversion. Frequency conversion can be inefficient, and can require a high peak power laser to provide sufficient light to the frequency conversion crystal.
Generating light at wavelengths below 213 nm, and especially below 200 nm can be very challenging. Light sources used for semiconductor inspection require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent. The lifetime and stability of current DUV frequency converted lasers is generally limited by the frequency conversion crystals, especially those exposed to DUV wavelengths like 266, 213, 200, and 195 nm.
High efficiency is important for a DUV laser. High efficiency allows a lower power fundamental laser source that is more reliable, smaller, and produces less heat. A low power fundamental laser will produceless spectral broadening if a fiber laser is used. Higher efficiency also tends to lead to lower cost and better stability. For these reasons efficient frequency conversion to the DUV is relatively important.
High speed inspection requires a light source with a continuous in time temporal profile or a profile pulsed at a very high repetition rate. Such a light source is required because each imaged position on the sample desirably exhibits the same or similar intensity. A similar intensity is provided by, for example, a continuous light source. Similar intensity may also be provided if the light source is pulsed with a repetition rate high enough such that many pulses are averaged at each imaged position. In this case the pulsed nature of the laser generally will not effect the measurements.
The requirements for high efficiency frequency conversion and high speed inspection indicate the laser light source preferably operates in a pulsed mode at relatively high repetition rates. In this way, high conversion efficiency and high speed inspection can be obtained from the light source.
In this environment, a laser based illuminator or illuminating arrangement having wavelengths that include ultraviolet-deep ultraviolet (UV-DUV) wavelengths may provide benefits over previous types of illuminators. Such an illuminating arrangement operating at sufficient power levels, providing adequate SNRs, that are relatively inexpensive and have relatively long lifetimes may be preferable to other illuminators in certain advanced applications.
It would therefore be desirable to offer a light source that effectively and efficiently generates light energy at wavelengths at or below 213 nm, using materials that have been processed specifically to provide increased lifetimes, stability, and damage thresholds over designs previously available.